Atom

Atoms are the smallest particles of an element which can take reaction.

Size of an atom: Atomic radius is measured in nanometres.

Atomic radii of hydrogen atom = 1 × 10–10 m.

Molecule

It is the smallest particle of an element or compound which can exist independently.

• Molecules of an element constitute same type of atoms.
• Molecules may be mono-atomic, di-atomic or polyatomic.
• Molecules of compounds join together in definite proportions and constitute different type of atoms.

 Dalton’s Atomic Theory

1. Every matter is made up of very small or tiny particles called atoms.
2. Atoms are not divisible and cannot be created or destroyed in a chemical reaction.
3. All atoms of a given element it are same in size, “mass and chemical properties.
4. Atoms of different elements are different in size, mass and chemical properties.
5. Atoms combine in the ratio of small whole number to form compounds.
6. The relative number and kinds of atoms are constant in a given compound. 9

Atomicity

The number of atoms constituting a Molecule is known as its atomicity.

Ions

The charged particles (atoms) are called ions, they charge or negative charge on it:

Negatively charged ions called anion (C1^�).

Positively charge ion is called cation (Na⁺).

Valency

The combining capacity of an element is known as its valency: Valency is used to find out how atom of an element will combine with the atom of another element to form a chemical compound.
To get in-depth knowledge of atoms and molecules the students can refer to atoms and molecules class 9 notes pdf and atoms and molecules class 9 ncert solutions pdf for further clarity.

NCERT Solutions for Class 9 Science Chapter 3 – Atoms and Molecules